One of the well-known mass-analyzing methods using an ion trap mass spectrometer or other apparatuses is an MS/MS analysis (or tandem analysis). In a general MS/MS analysis, an ion species having a specific mass (mass-to-charge ratio m/z) is first selected as a precursor ion from ionic species produced from a sample including the substance to be analyzed. Next, the precursor ion thus selected is dissociated into product ions by a CID (Collision Induced Dissociation) process. This is the first selection/dissociation operation of the ionic species. The product ions thus created are separated according to their mass and detected as in a normal MS analysis. Ion species derived from some kind of substances show characteristic dissociations by CID process; hence it is possible to obtain the information on the chemical structure or other properties of the substance to be analyzed by focusing on the portion where the dissociation took place for example.
In recent years, such mass spectrometers are being used more and more in the fields of biochemistry, medical care or pharmaceutical chemistry, and other fields. Accordingly, substances having larger molecular weight than before, e.g. protein, peptide, lipid, are becoming analyzed. Their chemical structure (composition) also tends to become more complicated. Hence, depending on the quality of the substance to be analyzed, ions often are not dissociated to have a sufficiently small mass by a single selection/dissociation operation. In such cases, one effective analysis method would be an MSn analysis (n≧3) in which a selection/dissociation operation is repeated more than once and the product ions finally generated are mass analyzed. If the selection/dissociation operation is performed only once in an MS/MS analysis, as described earlier, it is called an MS2 analysis. That is, in an MSn analysis, the selection/dissociation operation is repeated n−1 times.
In such an MSn analysis, it is necessary to previously determine, before beginning an analysis, an ionic species (precursor ion) as a target for each of the first, second, . . . , and (n−1)th selection/dissociation operations. Conventionally, in general, such selection of a precursor ion for each stage is often performed by the analyzer by analyzing the data obtained through the analysis. Hence, in the case where an MSn analysis with multiple stages of ion species' selection/dissociation operation is performed, the analyzer has to analyze the same sample many times while considering and changing the analysis condition. This is a very troublesome operation and lowers the analytical efficiency.
In order to ease such a troublesome operation, some known mass spectrometers have the function of automatic selection of a precursor ion in an MSn analysis. In the mass spectrometer described in Patent Document 1, for example, from a plurality of peaks appearing in a mass spectrum obtained by an MSn-1 analysis, an ion species corresponding to a peak with the highest signal intensity is selected as the precursor ion for the (n−1)th ion species' selection/dissociation operation in an MSn analysis. In the mass spectrometer described in Patent Document 2, a precursor ion for the (n−1)th ion species' selection/dissociation operation in an MSn analysis is determined in view of not only the peak having the highest signal intensity among a plurality of peaks appearing in a mass spectrum obtained by an MSn-1 analysis, but also other peaks.
Such mass spectrometers having an automatic precursor ion selection function save the analyst the trouble of examining the analysis data each time to select the precursor ion. However, each of such mass spectrometers determines a precursor ion based on the information of a single mass spectrum. Therefore, for example, even in a case where it is known that a specific fragment is desorbed by a dissociation, it is not possible to focus attention on an ion generated by the desorption and select it as a precursor ion in an analysis.
On the other hand, conventionally-known triple stage quadrupole mass spectrometers capable of an MS/MS analysis employ measurement methods such as a precursor ion scan for detecting all the precursor ions that produce a specific product ion, and a neutral loss scan for detecting all the precursor ions that desorb a specific neutral fragment (neutral species). The feature of such methods is to specifically detect only a precursor ion having a particular product ion and/or a neutral loss. However, such mass spectrometers cannot perform an MSn analysis of n≧3, and therefore cannot collect the sufficient information necessary for a structural analysis. In addition, if an ion is multivalent, accurate information cannot be obtained because it is sometimes difficult to distinguish different kinds of fragments desorbed from an ion, although this problem does not occur when the ion is monovalent.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-171442
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2004-257922